The invention relates to a method for measuring a magnetic field, particularly the earth's magnetic field, by means of a magnetic probe in the form of a coil with an iron core. A variable exciting current is applied to the coil having a magnitude such that the iron core achieves a positive and a negative saturation value. A change of the inductivity curve in comparison to a curve of the exciting current is determined by means of measuring the induced voltage at the coil. A strength of the external magnetic field in the direction of the axis of the probe is derived from this data. Also, the invention relates to a circuit arrangement for the implementation of this method.
Magnetic probes are particularly well suited for the precise determination of the relatively weak magnetic field of the earth. Given a known method for field determination by means of a magnetic probe (German patent application No. P 29 49 815, incorporated herein by reference), an alternating current is conducted through the winding of the probe which is so strong that the iron alternately achieves the positive and the negative saturation value. A voltage whose positive and negative half waves are of identical size (as long as no external magnetic field is influencing it) is then induced in a secondary winding. If, however, the external magnetic field of the earth is superimposed on the internal magnetic field of the probe, then the saturation will occur earlier or later depending upon the direction of the earth's magnetic field, and thus the induced secondary voltage will become asymmetrical. The differing amplitude of both half waves is a value from which the external field strength can be measured.
The method is based on the non-linearity of the iron. Therefore, the induced output voltage shows a significantly different curve shape than the input current. In particular, a large number of modulation or level-control dependent harmonics are generated which superimpose to form peak voltages of different size and thus vitiate the precise comparison of the two half-wave amplitudes. In order to keep this effect small, complicated and involved filters and amplifiers are necessary. A further problem of the known method is based on the fact that the generated signal is a voltage dependent on the field strength which must be generally digitized with an additional device for further processing.